It is known that rock, concrete, and the like can be fractured by applying an expansion force from within a borehole. Forces in the region of 10,000 to 15,000 lbs. per square inch over an adequate surface area will usually be sufficient for the purpose, although clearly much higher forces can be developed if required. In the past, proposals have been made for developing expansion forces mechanically. However, such systems were complex and prone to failure. In addition, since in many cases the forces which they developed tended to be localized, instead of spread around 360 degrees, and over a sufficient area, the forces were not developed in a manner which was adequate to fracture the rock.
Hydraulic devices have been proposed, but are either complex in design, or are apparently unsatisfactory for other reasons. One of the particular problems in developing a hydraulic device is that generally speaking such devices are based upon the use of an expandable sleeve or bladder, which is located on a tubular body, and in which the bladder is retained on the tubular body by means of end flanges. Hydraulic fluid is forced via the tubular body into the bladder, typically at pressures up to 10,000 to 12,000 psi. These pressures cause the bladder to expand outwardly into contact with the rock. The actual force applied to the rock is a function of the square inch area of the bladder surface, and may be as much as one million pounds of force, which is usually sufficient to fracture the rock. However, substantial forces are also applied to the two end flanges attached to the tubular body.
In the case of, for example, a three inch borehole requiring an expansion member of almost three inches diameter, the two end flanges may represent a relatively substantial surface area. When this surface area is subjected to a pressure of, say 10,000 psi, it will be appreciated that there may be a very substantial total axial force applied to each of the end flanges which may be in the region of 80,000 to 100,000 pounds. The tubular body, and the means whereby the end flanges are attached to it must thus be designed and engineered to withstand these very high axial forces.
In the case of expansion devices for smaller diameter boreholes, while the total force applied to each of the end flanges may be somewhat less, it will be appreciated that the diameter of the internal tubular body will also be less. Many steels will not withstand these high axial stresses.
A further problem in earlier designs of expansion devices, was the design of the bladder. In many cases, the bladder was of a relatively complex design, requiring special moulding techniques. Typically such bladders are made of a tough resilient flexible thermoplastic material. Polyurethane materials are suitable, and other specialized thermoplastics are also suitable. In each case, however, it is preferable that the bladders shall be formed in a mould, either by injection moulding, or casting, or the like, so as to ensure that they are of substantially identical dimensions, and can be produced at a reasonable cost. Bladders of a special design may require costly tooling and expensive moulding techniques.
Another problem in earlier designs, arises again from the design of the bladder or sleeve. It is necessary to seal each end of the bladder against the escape or extrusion, of hydraulic fluid, between the ends of the bladder, and the end flanges. Various different proposals have been made, none of which were entirely satisfactory. In addition, in several prior designs, the design required a substantial space between the central tube, and the interior of the bladder. This space must be filled with hydraulic fluid before the device can apply force to the rock. Thus it will take a considerable period of time for pumping of hydraulic fluid into the space. The requirement for a substantial volume of hydraulic fluid within the device will also reduce the efficiency of the device, due to the compressibility of the fluid. While in theory hydraulic fluids are incompressible, in practice, at these higher pressures, such fluids exhibit a relatively significant degree of compressibility. Since compressibility is obviously a function of the total volume of fluid within the device, it will be appreciated that efficiency will be greatly reduced if an excessive volume of fluid is present.
In addition, the space must be completely vented of air before the device will develop its full force potential.
Another factor in earlier designs is that due to the provision of the substantial space between the bladder and the tube, the diameter of the tube is substantially reduced, thereby reducing its ability to withstand axial stresses.
Another significant factor arises from these same considerations. Where the bladder defines end flanges of a significant area, enclosing a substantial hydraulic volume, then the abutting end flanges on the rod also define a substantial annular surface area at each end of the device. This annular surface area is exposed to the hydraulic pressure developed within the bladder. It will be appreciated that the larger the surface area of these end flanges, the greater will be the axial force developed. The force will be a function of the internal pressure, multiplied by the surface area of the end flanges. It will be appreciated, therefore, that increasing the area of the end flanges and thereby reducing the diameter of the internal tube rapidly reaches critical proportions. The smaller the diameter of the internal tube, the smaller will be its ability to resist axial stresses. Conversely, the greater the area of the end flanges, the greater will be the axial forces developed in use.
As a result, it is apparent that it is highly desirable to increase the diameter of the central rod and, at the same time, reduce the area of the end flanges.
When this is understood, it will also be appreciated that when the diameter of the central rod is maximized, and the area of the end flanges is minimized, the hydraulic volume within the sleeve or bladder which must be filled each time it is used will be reduced to a minimum. This will also reduce the cycling time of the device and reduce the time required to operate the pump, or other pressure device used to provide the hydraulic pressure, and will also minimize compressibility problems.
It is also a desirable feature if such devices can be connected together in tandem, to provide expansion forces over a greater axial distance.